Toast nigella, mustard and fennel seeds in your preserving pan and set aside.
Place tomatoes in bowl of boiling water. After 1 minute take the tomatoes from the water and carefully remove their skins.
Cut tomatoes into rough dice, removing any hard stalks or tough centres. Dice onions into similar sized pieces as the tomatoes.
Add tomatoes and onions, along with garlic, ginger and chopped fresh chillies, to pan and stir.
Bring to the boil and simmer for 30 minutes until you have a thick pulp.
Add salt, Kashmiri chilli powder and toasted spices along with half the vinegar and simmer for another 30 minutes.
Add the remaining vinegar and the sugar. Stir well to ensure the sugar has dissolved.
Simmer for 1 to 1½ hours until thick and glossy. Draw a spatula through the chutney, if the base of the pan remains visible for a few seconds, it’s done.
Whilst chutney is still hot, ladle into sterilised jars and fill to within 6mm (1/4″” inch) of the top. Use the back of a spoon to pack down your chutney and remove any air pockets.
Seal tightly with a lid and label when cold.